The leading cause of death in the US and most other developed nations is heart disease, whose incidence rises sharply with age. The heart muscle has a high requirement for energy, most of which is generated by oxidative reactions that are carried out in mitochondria. Recently, deletions and point mutations of the mitochondrial DNA (mtDNA) have been associated with sporadic and familial cardiomyopathy. Using sensitive new technologies, the identical mtDNA deletions that cause cardiomyopathy in persons with genetic disease can be detected at a lower frequency in normal individuals. In normal individuals, the frequency of the deletions rises strikingly in human,heart tissue with age, by more than 1000- fold. Individuals with atherosclerosis have 10 to lOO fold higher levels of this deletion than normal individuals. One possible inference is that atherosclerosis induces oxidative stress which induces mtDNA deletions; conversely individuals with higher deletion levels might be more prone to atherosclerosis. In order to lay the groundwork for experimental studies into the cause/effect relationships of mtDNA deletion and point mutations and heart disease, three specific aims are proposed: (1) to develop and validate assay systems for the detection and quantification of mtDNA deletion mutations in the laboratory rat and laboratory mouse, (2) to test the association of particular deletions and point mutations of mtDNA with particular cardiomyopathies in human autopsy samples, (3) to address the correlation of dysfunctional areas of human heart tissue with mtDNA structure by the SURF technique, and (4) to develop an improved assay for rare point mutations, with which to determine whether or not rare point mutations accumulate in human heart mtDNA with age in humans and rodents.